Receptor-ligand interactions and receptor transmembrane signalling are presently poorly understood, and almost no structural information is available. We are using X-ray crystallography to investigate these processes at the atomic level. During the last run, we were able to collect two complete data sets of the growth hormone-receptor complex (mutant) and we would like to refine the structure of the growth hormone-prolactin receptor complex to an improved resolution of 2.6 w from 2.9 w.